In the octagonal halls to the south, the leftmost octagon has a green button on the south wall near the vent. Use it to raise some steps to the vent, allowing you to reach some rockets and a rocket launcher. (sector 233)

At the rightmost hallway with former humans, go towards the lower right column and look at the side with slits in it. Open the column from this side to get an invulnerability. (sector 338)

From the previous secret, go all the way across the bridge to the teleporter, then head directly south and open the wall here for a teleporter taking you directly to the soul sphere in the middle of the entrance. (sector 353)